Among generally used powder coatings including epoxide-, acrylate- and polyester-based coatings, polyester-based powder coatings are known to form paint films having excellent properties.
Powder coatings are used in various fields including household electrical goods, automobiles, building materials and the like. From the aesthetic point of view, the coated surface of the paint film is required to be glossy (60.degree. specular gloss, about 90 to 100%), mat (60.degree. specular gloss, about 30% or less) or semi-mat (60.degree. specular gloss, about 30 to 70%).
In general, such a mat or semi-mat powder coating is prepared by a process in which a rough particled pigment is used as an additive agent in a large quantity or by a process in which a polyester powder coating and an acrylic powder coating are dry-blended (JP-B-61-19668). (The term "JP-B" as used herein means an "examined Japanese patent publication")
However, the addition of a large quantity of a rough particled pigment reduces the smoothness and mechanical strength of the resulting paint film. Also, the latter process in which two types of powder coatings are dry-blended has disadvantages in that a mat or semi-mat coating cannot be obtained in a single step, thus entailing high cost for the production of the coating, as well as non-uniformity of the blend and change in the blending ratio at the time of recycle use.
With the object of overcoming such problems, a resin composition for use in powder coatings has been proposed in JP-A-64-1770 which comprises a polyester having a hydroxyl value of 1,200 g eq/10.sup.6 g or more, another polyester having a hydroxyl value of from 200 to 1,000 g eq/10.sup.6 g and a curing agent of the blocked isocyanate type, where the difference in the gel times between these two polyesters is at least 3 minutes. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.)
However, the preparation of a powder coating using this prior art resin composition requires to use a great amount of the curing agent, which has only 2 to 3 functional groups, thus entailing high cost for the production thereof. Further, this prior art has disadvantages in that the blocking agent scatters and pollutes a baking oven when the polyesters are allowed to react with the curing agent of the blocked isocyanate type, and about 10% weight loss occurs at the same time.
These problems have been attempted to be settled by the use of tetramethoxymethyl glycoluril curing agent of a tetrafunctional compound but the curing agent has not functioned sufficiently due to absence of a suitable catalyst for this curing agent.
In view of the above, the present invention is directed to overcoming the aforementioned problems involved in the prior art, thereby providing an inexpensive resin composition for powder coating use which does not pollute a baking oven, which shows less weight loss at the time of reaction and which can form a stable semi-mat or mat paint film.